circular_q.h

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// Copyright(c) 2015-present, Gabi Melman & spdlog contributors.
// Distributed under the MIT License (http://opensource.org/licenses/MIT)
 
// circular q view of std::vector.
#pragma once
 
#include <vector>
#include <cassert>
 
namespace spdlog {
namespace details {
template<typename T>
class circular_q
{
    size_t max_items_ = 0;
    typename std::vector<T>::size_type head_ = 0;
    typename std::vector<T>::size_type tail_ = 0;
    size_t overrun_counter_ = 0;
    std::vector<T> v_;
 
public:
    using value_type = T;
 
    // empty ctor - create a disabled queue with no elements allocated at all
    circular_q() = default;
 
    explicit circular_q(size_t max_items)
        : max_items_(max_items + 1) // one item is reserved as marker for full q
        , v_(max_items_)
    {}
 
    circular_q(const circular_q &) = default;
    circular_q &operator=(const circular_q &) = default;
 
    // move cannot be default,
    // since we need to reset head_, tail_, etc to zero in the moved object
    circular_q(circular_q &&other) SPDLOG_NOEXCEPT
    {
        copy_moveable(std::move(other));
    }
 
    circular_q &operator=(circular_q &&other) SPDLOG_NOEXCEPT
    {
        copy_moveable(std::move(other));
        return *this;
    }
 
    // push back, overrun (oldest) item if no room left
    void push_back(T &&item)
    {
        if (max_items_ > 0)
        {
            v_[tail_] = std::move(item);
            tail_ = (tail_ + 1) % max_items_;
 
            if (tail_ == head_) // overrun last item if full
            {
                head_ = (head_ + 1) % max_items_;
                ++overrun_counter_;
            }
        }
    }
 
    // Return reference to the front item.
    // If there are no elements in the container, the behavior is undefined.
    const T &front() const
    {
        return v_[head_];
    }
 
    T &front()
    {
        return v_[head_];
    }
 
    // Return number of elements actually stored
    size_t size() const
    {
        if (tail_ >= head_)
        {
            return tail_ - head_;
        }
        else
        {
            return max_items_ - (head_ - tail_);
        }
    }
 
    // Return const reference to item by index.
    // If index is out of range 0…size()-1, the behavior is undefined.
    const T &at(size_t i) const
    {
        assert(i < size());
        return v_[(head_ + i) % max_items_];
    }
 
    // Pop item from front.
    // If there are no elements in the container, the behavior is undefined.
    void pop_front()
    {
        head_ = (head_ + 1) % max_items_;
    }
 
    bool empty() const
    {
        return tail_ == head_;
    }
 
    bool full() const
    {
        // head is ahead of the tail by 1
        if (max_items_ > 0)
        {
            return ((tail_ + 1) % max_items_) == head_;
        }
        return false;
    }
 
    size_t overrun_counter() const
    {
        return overrun_counter_;
    }
 
private:
    // copy from other&& and reset it to disabled state
    void copy_moveable(circular_q &&other) SPDLOG_NOEXCEPT
    {
        max_items_ = other.max_items_;
        head_ = other.head_;
        tail_ = other.tail_;
        overrun_counter_ = other.overrun_counter_;
        v_ = std::move(other.v_);
 
        // put &&other in disabled, but valid state
        other.max_items_ = 0;
        other.head_ = other.tail_ = 0;
        other.overrun_counter_ = 0;
    }
};
} // namespace details
} // namespace spdlog